You could try this, Antonis. I don't know the cultural considerations but it seems that you'd be giving away the farm by stating you be "
at his disposal". Maybe it's simply a stated cultural nicety that both parties wouldn't abuse.
Dear Sir?Madam,
I would like to thank you for your kindness during the negotiations for our new rental agreement. Please feel free to contact me with any concerns you might have regarding the property.
Sincerely,
Antonis